Hoard all those old songs, e-books, and applications. A new patent for Amazon, and a start-up in Cambridge, Mass., point to a possible coming era of used content sales.

In late January, Amazon received patent No. 8,364,595 for a "secondary market in digital objects." The patent describe an electronic marketplace for used e-books, music, video, computer applications and other digital content that is transferred by moving, streaming or download to another digital .

In Amazon's vision, after the content is transferred, the original content is deleted from the seller's storage. A digital object is allowed a certain number, or a threshold, of moves and downloads for a given owner, and when that number is reached the owner's rights to transfer expire.

'Future of the Digital Space'

In addition to being a sale, the transfer also could represent a rental, a gift, a loan or a trade. This approach, the patent says, maintains "scarcity" of the digital object by limiting the number of copies, and it implies there are an expanded set of rights obtained by the digital object's owner. "A 'used' digital object is one to which a user has legitimately obtained access or ownership rights," the patent said, "which the user may permissibly transfer to another user."

A start-up called ReDigi, based in Cambridge and launched in 2011, is already undertaking an actual used digital content market. ReDigi analyzes the material you want to sell, such as a song, to make sure it was purchased and not simply copied from some source. The file is then uploaded to ReDigi's , and a ReDigi desktop program on the seller's makes sure the original is erased and no additional copies are made.

ReDigi is currently being sued by Capitol Records for copyright infringement, but ReDigi claims its approach is legal because the first-sale doctrine allows an owner to resell a purchased item of intellectual property, such as a book or CD. On Wednesday, Capitol was denied a preliminary injunction to shut down ReDigi, although the record company's suit is moving forward.

Earlier this week, ReDigi responded to Amazon's patent by saying that, while the Amazon patent provides "further proof that the secondary market is the future of the digital space," its model is "significantly different" from Amazon's.

'Transfer,' Not 'Copy'

ReDigi said that Amazon's patent describes a " 'copy and delete' mechanism," and said that this approach has been central to the "music and publishing industries' skepticism and opposition to a 'used' digital marketplace."

By contrast, ReDigi said, its advanced technology uses a "transfer only mechanism" that first verifies the digital object is "legally eligible for resale," and then "only the 'original' good is instantaneously/atomically transferred from seller to buyer without any copies."

While ReDigi does not clarify how its "transfer" is not a "copy" but Amazon's transfer is, it adds that, unlike Amazon's model, there is "participation of all parties from consumer to artist/author to copyright holder" in the profit chain.

Melissa Webster, program director for content technologies at IDC, called the idea of a user digital content marketplace "a very intriguing concept." She noted, however, that the intellectual property laws for digital content provide "no clear picture" of what might be acceptable, plus different categories of content have different and often complex terms and conditions attached to their sale.

Webster said that, as an experiment about a year ago, she tried to sell on eBay a used application, and was confronted with a variety of conditions, rights limitations and forms to navigate. But, she noted, if the software vendors and other rights holders got a piece of the resale, perhaps the rights structures will be simplified to accommodate such a market.